An optical modulator used for display optical modulation must have sufficient resistance (hereinafter, optical resistance) to optical damage, for example, because optical modulation is performed for high-output light of 100 milliwats or more.
In order to obtain the optical resistance, in principle, an acousto-optic modulation element may be used, which can modulate an optical beam of a relatively large diameter (beam diameter is about several tens of micrometers to several hundreds of micrometers).
Patent Literature 1 describes an acousto-optic element. FIGS. 1A and 1B show a configuration of the acousto-optic element. The acousto-optic element modulates light by changing a refractive index in acousto-optic crystal 10 based on an acoustic wave transmitted through acousto-optic crystal 10 to cause Bragg diffraction. The configuration described in Patent Literature 1 employs a method for reducing, without forming any antireflection film having low optical resistance, a reflection component of the light by entering the light into acousto-optic crystal 10 at a Brewster's angle. Specifically, the acousto-optic element has incident surface 11 and exit surface 12 of light formed in parallel to each other. In the acousto-optic element, a center line of electrode 13a having piezoelectric element 13 almost matches the traveling direction of the light so that light incident on entrance surface 11 at the Brewster's angle can be effectively diffracted by an ultrasonic wave traveling through acousto-optic crystal 10.
There have been proposed many optical switches for performing light switching by applying an electric field to a crystal having an electro-optic effect (hereinafter, electro-optic crystal) to cause a change in refractive index. For display optical modulation, for the abovementioned reason, the preferred way is that an optical switch capable of modulating an optical beam having a relatively large beam diameter and having high optical resistance be provided.
Patent Literature 2 describes an optical switch that uses Bragg reflection. FIG. 2 shows a configuration of the optical switch.
As shown in FIG. 2, the optical switch includes optical waveguide layer 21 having an electro-optic effect, and first and second electrode groups 31 and 32 located in optical waveguide layer 21. Each of first and second electrode groups 31 and 32 includes a plurality of plate-shaped electrodes 30 extending in parallel to a thickness direction of optical waveguide layer 21. Plate-shaped electrodes 30 are arranged at equal intervals. A section of a plane of each of first and second electrode groups 31 and 32 intersecting the thickness direction of optical waveguide layer 21 is formed into a comb shape, and plate-shaped electrodes 30 equivalent to teeth of the combs are alternately arranged.
In the abovementioned optical switch, applying a voltage between first and second electrode groups 31 and 32 causes a change in the refractive index in the area between adjacent plate-shaped electrodes 30. As a result, a cyclic refractive index change occurs in optical waveguide layer 21. A portion where the cyclic refractive index change occurs functions as a diffraction grating, and incident light is subjected to Bragg diffraction. On the other hand, when application of the voltage to first and second electrode groups 31 and 32 is stopped, the portion stops functioning as the diffraction grating, and hence the incident light is transmitted through an area between plate-shaped electrodes 30. The electro-optic element of this structure allows free selection of a thickness of optical waveguide layer 21 through which the light is guided. Thus, even an optical beam of a relatively large beam diameter can be modulated, and high optical resistance can be achieved.